The present invention relates to methods and compositions for use in subterranean cementing operations. More particularly, the present invention relates to cement compositions comprising degradable materials, and methods of using such compositions in subterranean cementing operations.
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into an annular space between the walls of a well bore and the exterior surface of a pipe string disposed therein. To ensure that the annular space is completely filled, a cement slurry is pumped into the annular space until the slurry circulates to the surface. The cement composition is then permitted to set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement. The hardened cement substantially supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also used in remedial cementing operations, such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
Subterranean formations traversed by well bores naturally may be weak, extensively fractured, and highly permeable. In some cases, if the fracture gradient of the formation is exceeded by the hydrostatic head pressure normally associated with cement pumped into the well bore, the formation will fracture. This may result in the loss of cement into the extensive fractures of the formation. This can be problematic because, inter alia, less cement composition will remain in the annular space to form the protective sheath that bonds the pipe string to the walls of the well bore. Accordingly, loss of circulation of the cement slurry into the formation is of great concern.
Conventional attempts to solve the problem of lost circulation of cement slurries include adding polymeric flakes or film strips that may bridge the cracks and fractures in the formation and, thus, prevent the loss of the cement slurry. Examples of such materials include cellophane flakes, polypropylene flakes, or mica flakes, among others. However, the use of polypropylene flakes may be undesirable because the polymer is not biodegradable. Mineral flakes such as mica often have unsuitable sizes that preclude their use.
Conventional attempts to solve the problem of inadvertently fracturing the subterranean formation during cementing operations have also involved, inter alia, the use of cementing slurries with reduced densities. For example, cement slurry densities can be desirably reduced by incorporating an expanding additive, such as nitrogen, into the cement composition. Alternatively, lightweight particulate additives, such as hollow glass or ceramic beads, may be incorporated into the cement composition at the surface. However, these methods may be problematic because, inter alia, they can require elaborate and expensive equipment, which may not be accessible for use in remote areas.
Certain conventional cement compositions also may become brittle and/or inelastic at some point after setting into a cement sheath. This may be problematic because, inter alia, an excessively brittle or inelastic cement sheath may become unable to provide desired zonal isolation, and may require costly remediative operations. This may be particularly problematic in the case of multilateral wells. If the cement sheath in the area of the junction between a principal well bore and a lateral well bore in a multilateral well is excessively brittle or inelastic, it may be unable to withstand impacts that may occur, e.g., when tools used in drilling and completing the well collide with casing in the junction area as the tools are moved in and out of the well.
Well bores that comprise an expandable tubular present another scenario where an excessively brittle or inelastic cement sheath may be problematic. The expansion of an expandable tubular inadvertently may crush at least a portion of the cement sheath behind the tubular, thereby impairing the cement sheath's ability to provide the desired zonal isolation.